How to create/make UV coordinates for polyhedron
You can unwrap the tetrahedron in one of 2 ways:


You can create seams (CTRL+E > Mark Seams) running along the 3 edges from the base to the tip. Then select all vertices and press U > UV Unwrap.






You can create seams on all edges  (CTRL+E >Mark Seams) , and overlay each uv island on top of itself. Then select all vertices and press U > UV Unwrap.





Please note the tetrahedron shape in my example is an approximate shape only in order to demonstrate the unwrap technique.